An image or pattern that is to be displayed on a display is typically defined by a series of drawing instructions contained in an application program. A drawing instruction may be, for example, an instruction to display a rectangle at a particular location on the display. To draw an image on a bit-mapped display means that the drawing instruction has to be converted into respective picture element (pixel) locations. The process of converting a drawing instruction into respective pixel locations is not a trivial exercise and may consume an inordinate amount of time to do so when the image is complex.
In addition, a complex image may be defined in a so-called memory-resident pixmap before it is copied to a screen (frame) buffer for display. This is done so that the complete image may be displayed in one frame interval, rather than over a series of frame intervals, as would be the case where a pixmap is not used. To define an image in a memory-resident pixmap means that a computer, such as the host processor in a workstation, converts drawing instructions into pixmap locations representative of respective pixel locations. Consequently, the response time of a host processor, and hence the workstation, deteriorates whenever the host processor is converting drawing instructions representative of a complex image into memory-resident pixmap locations.